I have a lot of them. I think my favorite characters to draw are determined by how often I make illustrations of them for fun. There are some that I draw like, all the time, and some that I really need to get in the mood to squeeze out a measly illustration of them. What kind of illustration I put them into also tends to be consistent for each character.
With that in mind, here they are:

Tao. I usually draw her in illustrations that are “just for fun”, with little meaning to them (sometimes a little foreshadowing though, like this one. I won’t tell you what it’s foreshadowing)

Mina. She isn’t my favorite character per se, but I draw her a lot, especially in this dress because it carries a lot of symbolism. I tend to put quite a bit of foreshadowing into illustrations with her. She’s also one of the only characters I actively made a character sheet for.

Iseul (the girl on the right). She’s just fun to draw, and one of my favorite characters. I also put a crazy spoiler in her design, but no one I asked has guessed it right yet. Heck, even her color palette is symbolic.

Seoyun. The guy on the previous drawing is her brother and one of the main characters, but I still draw her more even though she only appears in flashbacks (she’s dead
). Obviously, the illustrations she’s in are usually extremely symbolic and foreshadowing.

Elsa. Usually in symbolic illustrations, but I sometimes do some silly doodles with her too. I draw her whenever I feel weirded out by something in my life. In fact, I gave her many of my own thoughts and ideas.

And lastly, Rei. I just love doodling her all over my stuff.